In composites manufacturing, the object is to produce the highest quality part possible. A common defect in composites is the presence of voids and porosity. Voids and porosity can be minimized by minimizing the sources of gas leading to such voids and porosity, and maximizing the removal and reduction of gas volume. Sources of the gas include but are not necessarily limited to entrapped air during preparation of the part, entrapped moisture, and generation of gases as a by-product of the chemical reactions during cure of the matrix, and leaks during the manufacturing process. Regardless of the method of composites manufacturing, the removal of gas by the application of vacuum to the part is a well-established procedure. It is generally accepted that a sufficient vacuum must be pulled on a part to achieve good quality, and considerable time and effort is expended in managing and controlling the vacuum pulled on a part.
Current methods for assessing the vacuum pulled on a part in the most part involve the measurement of the gauge or absolute pressure in the vacuum lines. The lower the measured pressure, the better the vacuum pulled on the part. As it is known that the pathways into the part and within the part may be blocked, a variety of methods are used to ensure that there are open pathways to every position in the part. Methods include but are not limited to multiple vacuum lines, so-called ‘breather’ fabrics, and specialized raw composite material forms such as so-called ‘out-of-autoclave’ pre-impregnated materials.
Leaks are a particularly common source of quality problems. Sources of leaks include but are not limited to improper sealing of seams between mould segments, improper sealing of flexible vacuum bags over the part and mould, puncturing of the flexible vacuum bag, failure of rubber and other hollow inserts, and failure of the solid mould or semi-flexible caul plate. Leaks can be present from the beginning of the manufacturing process, or can occur later in the process, for example when heating or pressurizing the assembly inside an autoclave, oven, or press.
The most common methods for detecting the presence of leaks, or alternatively confirming the vacuum integrity of the system, consist of shutting off the evacuation of the assembly, and monitoring the rise of pressure as air leaks into the assembly. An increase in pressure less than a prescribed amount, over a prescribed time, is considered to be indication of adequate vacuum integrity. This procedure is time-consuming, particularly if a leak is found to be present, and the procedure must be repeated after every attempt to fix the leak. Consequently the conventional leak detection method may take many hours to locate a single leak, causing significant and costly manufacturing downtime. Furthermore, the procedure is sensitive to the size of the part, and the larger the part, the less sensitive the procedure. Other methods, including sensitive coatings on the vacuum bag, and the use of mass flow sensors for vacuum assisted resin transfer moulding, have also been evaluated. Additionally, particularly in a very large part, there is currently no simple way to identify even the approximate location of the leak, other than by compartmentalizing the part. Outside of composites manufacturing, gas flow rates out of a system are often monitored and used instead of, or with, gas pressure measurements. A fundamental problem to date has been that the relationship between gas pressure and gas flow rates is complex.
Accordingly, there is a need for a system that provides continuous usable information on the evacuation state of the assembly (including during cure), and in the case of a leak, allows for immediate and continuous monitoring of the leak, including preferably guidance as to the location of the leak and its significance.
In the prior art applicant is aware of U.S. Pat. No. 3,818,752 which issued to Lindeberg on Jun. 25, 1974 for A Method And Apparatus For Testing Tightness. Lindeberg describes the use of pressure differential across a valve for detecting leaks in an enclosed volume wherein, after the pressure difference across the valve falls below a predetermined value, any further flow that is sensed is indicative of a leak in the volume. Lindeberg also discloses that if a leak is detected, the location of the leak may be obtained by smearing the volume tank with a soap solution or the like so as to provide an indication of where air or liquid is forced out of the tank.
Applicants are also aware of United States Published Patent Application, Publication No. US2008/0252470, which published on Oct. 16, 2008 in the application of Taricco entitled Leak Detectors and Leak Detection Methods. Taricco describes that it may be necessary in the field of composite structure manufacturing to leave a pressure or vacuum on a system for a prolonged period without loss of pressure or vacuum as a leak test. A vacuum sensing and alarm system is disclosed by Taricco as being used together with a pressure gauge, separate or integral with the system, wherein the pressure gauge would confirm that the pressure or vacuum remained within acceptable limits throughout the prolonged period. An adjustable pressure of vacuum switch detects a more rapid leak. Alternatively the pressure switch may be replaced by a pressure transducer.
Applicants are also aware of United States published patent application, Publication No. US2009/0273107 which published Nov. 5, 2009 in the application of Advani et al. entitled System and Method Of Detecting Air Leakage in A VARTM Process. Advani et al. disclose using heated air and distributing the heated air along an interface along a bagging film and the surface of a mould so as to locate leaks in the bagging film by determining the temperature distribution of the air along the interface. Advani et al. describe that during the VARTM Process that checking the vacuum level is standard procedure especially when a large part is being made, as any air leakage will decrease the part quality. Advani et al. further disclose that the primary leak isolation method which is conventionally used is performed by vacuuming the air out of the mould and if the vacuum pressure level has not decreased after a predetermined amount of time that the mould is considered to be free of air leaks, but that the disadvantage of this method is that it is only able to indicate if there is or is not a leak and does not specify the location of any leak. Thus Advani et al. teach using thermal leak detection employing a heat gun to warm air and introduce that warm air to potential leak areas around the mould, and that the method may also utilize a infrared camera to capture thermal images of the tested areas.
Applicants are also aware of United States published patent application, Publication No. 2010/0326584, which published on Dec. 30, 2010 in the application of Schibsbye entitled Method and Apparatus for Detecting Leak in A VARTM Process. Schibsbye discloses that during producing a composite structure, and in particular during the evacuation process, an air flow level through at least one vacuum outlet is measured. Schibsbye describes a method for manufacturing a composite structure which includes fibre reinforced material using a vacuum assisted resin transfer moulding process wherein the fibre material is impregnated with liquid resin. Schibsbye identifies a problem in the VARTM process wherein dry spots where the fibre material is not impregnated with resin provide areas for air pockets which need to be repaired. Schibsbye also identifies that leaks in the sealing between the mould part and the vacuum bag and/or in the vacuum bag itself may lead to problems with effectively evacuating the mould cavity or effectively filling the mould cavity with resin, thereby also being a cause of dry spots. Schibsbye states that even very small holes can cause these problems, and as fibre composite structures, such as wind turbine blades may have a length of 60 meters and have a surface area of several hundred square meters, that it can be very time consuming to find the leaks thereby prolonging the overall production time of the laminate structure.
Consequently, Schibsbye proposes the use of a gas mass flow sensor connected to an inlet tube in order to measure the gas flow through the interior of the container. A pressure transducer is provided so as to monitor the vacuum level, that is, the pressure in the interior of the sealed container and consequently the vacuum level of the mould cavity or the individual mould cavity sections. If it is determined that the gas flow for a given apparatus exceeds a predetermined threshold value for a given vacuum level, then the operator knows that a leak exists in the mould cavity, and if only a single apparatus identifies such a leak then it can be concluded that the leak exists in the corresponding mould cavity section. Schibsbye teaches that by using flow sensors an operator of the VARTM process can identify leaks and the location of such leaks faster.
Finally, applicants are aware of three United States published applications having a common inventor; namely, Miller, and a common assignee; namely, the Boeing Company. Thus in United States published patent application, Publication No. US2008/0148817, published Jun. 26, 2008, in the application of Miller et al. entitled Leak Detection In Vacuum Bags, the use of a leak detection film covering the inside face of the vacuum bag is disclosed wherein the film includes a gas permeable binder carrying oxygen sensitive material that changes in physical appearance at the location of an air leak. Miller et al. discuss that flexible vacuum bags are used in manufacturing such as the fabrication of composite structures and the bonding of parts, for example, in the aerospace industry where vacuum bags may be used in vacuum bag moulding wherein a flexible bag is placed over a part pre-form and sealed along a mould flange. Air is evacuated and liquid resin is drawn into the bag which is infused into the pre-form so that any leaks in the vacuum bag may allow air to enter and form bubbles in the resin matrix resulting in an unacceptable amount of porosity in the matrix.
Thus Miller et al. propose that leaks may be detected in gas impermeable, transparent membranes used to maintain a pressure differential by the use of a gas permeable film or coating placed on or near the membrane that emits or reflects light of various wavelengths in the area of the leak or pressure gradient so as to provide rapid visual detection of air leaks in vacuum bags. The gas sensitive material changes in appearance in response to exposure to gas caused by a leak in the bag.
In United States published patent application, Publication No. US2010/0170326, published Jul. 8, 2010, in the application of Miller et al. entitled Leak Detection In Composite Tools, which is a continuation-in-part of the aforementioned application to Miller et al., Miller et al. state that, although a vacuum integrity test may provide a means to indicate the presence of a leak, the vacuum integrity test may lack the capability to allow for identifying the location of leaks on the tool, and that another draw back is that the vacuum drop check may not provide an indication as to whether the leak is in the tool, in the vacuum bag, or in the seal that seals the vacuum bag to the tool. Miller et al. propose the use of a breather layer interposed between the tool and the leak detection film for facilitating air flow therebetween.
In United States published patent application, Publication No. US2011/0259086, published Oct. 27, 2011, in the application of Miller et al. entitled Leak Detection in Vacuum Bags, a device is described for indicating the location of an air leak in a vacuum bag used to process composite parts. The device includes a layer of material on the inner face of the bag that changes in appearance due to an oxidation-reduction reaction in the areas of the layer exposed to oxygen caused by a leak in the bag. Miller et al. describe the use of an ink or dye which is applied to the inner face of the vacuum bag film, wherein once a vacuum is drawn within the bag causing the air pressure within the bag to drop which then allows the atmospheric pressure to push the bag down onto the layup and to compact the layup, the colorimetric material is activated by directing ultraviolet light through the transparent vacuum bag and onto the ink rendering the ink reactive to oxygen so that the ink changes in color when exposed to oxygen due to a leak.